1. Field of the Invention
The present invention relates to an Electronic Article Surveillance System (hereinafter referred to as "E.A.S." system), and more particularly, to an E.A.S. system which is designed to radiate or emit a magnetic flux in a doorway to a store or the like so that a tag attached to goods is responsive to the magnetic flux in order to monitor whether the goods are being illegally taken out from the store without being processed by a cashier.
2. Description of the Prior Art
Such a known E.A.S. is generally made such that an antenna for magnetic flux radiation constructed with a plane coil is installed in a doorway to a store to continuously radiate a magnetic flux into the doorway while a tag incorporating a coil made to serve as a reception antenna is attached to all goods in the store so that the coil of the tag detects the magnetic flux therefrom in the doorway to issue an alarm in response to detection when goods unprocessed by a cashier are being taken out from the store.
Referring to the drawings, a description will be made hereinbelow of the relationship between a transmission antenna and a reception antenna in a prior art E.A.S. system.
FIGS. 9A and 9B are illustrations of a magnetic flux radiated from a transmission antenna and a configuration of the transmission antenna, FIGS. 10 to 12 are illustrations available for the explanation of the direction of a reception coil and a reception sensitivity, and FIG. 13 is an illustration useful for the description of a tag attached to goods. In FIGS. 9 to 13, designated at 1 and 2 are a transmission antenna and a reception antenna, respectively.
As shown in FIG. 13, a tag having a reception coil 2 and an alarm circuit is generally attached to all goods in a store through string or the like which is breakable and removable by the customer or shopper. When the customer takes the goods out of the store, the reception coil 2 of the tag detects a magnetic flux from a transmission coil, which will be described later, and activates the alarm circuit. Thus, the alarm circuit sounds an alarm to inform clerks. In addition, when the string is broken by the customer, or is disconnected therefrom, the alarm circuit is also capable of issuing an alarm.
The transmission antenna 1 is, as shown in FIG. 9A, of a loop antenna type constructed on a plane, and is disposed on a floor in the doorway of the store or elsewhere. The transmission antenna 1 emits, as a signal being inputted thereto, a magnetic flux indicated at G in FIG. 9A. The radiation area of the magnetic flux is limited to the area in which the customers go in and out. The reception coil 2, acting as a reception antenna set within the tag attached to the goods, is, as shown in FIGS. 10 to 12, constructed in a manner of winding a piece of conductive wire into a ring-like configuration. When the customer tries to take the goods out of the store, the reception coil 2 receives the magnetic flux G from the transmission coil 1 to cause the alarm circuit, not shown, to issue an alarm.
However, in the case of monitoring the pilferage or theft by a combination of the transmission antenna 1 being situated on or in the floor and the reception coil 2 being attached to the tag, there is a possibility that the reception coil 2 cannot sense the magnetic flux G from the transmission antenna 1 when the reception coil 2 assumes certain positions with respect to the goods taken out.
A description will further be made of the detection sensitivity depending upon the position of the reception coil 2 with respect to the magnetic flux G to be emitted from the transmission antenna 1 on the floor.
In FIG. 10, the X axis, being one axis passing through the center of the transmission antenna 1 on the floor, extends along the direction of the doorway. FIG. 10 shows the state in which the customer walks along the X axis and attempts to take the goods out of the store. Further, in this case, in the reception coil 2 within the tag, as shown, an axis perpendicularly intersecting a ring of the reception coil coincides with the Z axis normal to the floor surface. In FIG. 10, the reception coil 2 is drawn in a state that the ring constituting the reception coil 2 is cut off along the X-Z plane, and the actual configuration thereof is circular as shown in FIGS. 11 and 12.
Further, as shown in FIG. 10, when the axis of the reception coil 2 directs to the Z axis direction, as indicated by the bold lines, a number of magnetic fluxes G can cross the reception coil 2, with the result being that the reception coil 2 is able to sense the magnetic fluxes G with a high degree of sensitivity.
On the other hand, FIG. 11 shows the case in which the axis of the reception coil 2 turns to the X axis direction, i.e., extends along the direction of the doorway. In this case, the magnetic flux G crossing the reception coil 2 becomes zero when the reception coil 2 stands right above the transmission antenna 1. Further, when the reception coil 2 is slightly separated from the transmission antenna 1, the magnetic flux G slightly crosses the reception coil 2. Accordingly, in the case of an example as shown in FIG. 11, the reception coil 2 is able to detect the magnetic flux G with an intermediate sensitivity.
Moreover, if, as shown in FIG. 12, the axis of the reception coil 2 turns to the y axis direction, i.e., extends in a direction perpendicular to the direction of the doorway, then the magnetic flux G from the transmission antenna 1 fails to cross the reception coil 2 irrespective of the reception coil 2 being moved in any way within the X-Z plane. For this reason, in the example as shown in FIG. 12, the reception coil 2 is able to sense the magnetic flux D with only an extremely low sensitivity, or cannot detect it at all.
Although in FIGS. 10 to 12, the illustrations are made such that the reception coil 2 passes just above the X axis, in a case where the reception coil 2 is shifted from the X axis and is transferred in parallel to the X axis, even in the case as shown in FIG. 12, the reception coil 2 can detect the magnetic flux G, but with a low sensitivity.
As described before, an antenna system of antipilferage equipment based on the prior art suffers from the problem that because of the position of the reception coil working as the reception antenna within a tag attached to the goods, difficulty is encountered in detecting pilferage when the goods assumes a certain positions.